1. Field of the Invention
The present invention relates to a self-sustaining center-anchor microelectromechanical switch and a method of manufacturing the same and, more particularly, to a self-sustaining center-anchor microelectromechanical switch that driven by an electrostatic force used for controlling a RF signal in an electronic system for high frequency.
2. Discussion of Related Art
In order to control a signal in an electronic system with a high frequency bandwidth, an easily integratable semiconductor switch, such as a field effect transistor (FET) or a p-I-n diode, has been used, but since each semiconductor has problems, such as a high insertion loss, a low isolation loss, and a signal distortion, a research on the microelectromechanical switch has widely been progressed.
Generally, the microelectromechanical switch comprises a movement element that moves relative to a substrate, and a driving element that drives the movement element. The driving element has two electrodes that are located facing each other, and the movement element is configured to move in a horizontal direction or in a vertical direction to the substrate, or to rotate within a predetermined range of angle with respect to the substrate, and thus the movement element is driven according to the electrostatic force generated by the voltage applied to the driving element to perform a switching operation.
FIG. 1A is a plan view for illustrating an example of a cantilever type microelectromechanical switch of the prior art, and FIG. 1B is a cross-sectional view taken along line A1-A2 in the microelectromechanical switch of FIG. 1A. The cantilever type microelectromechanical switch of the prior art is disclosed in U.S. Pat. No. 5,578,976.
A lower electrode 2 and a signal line 3 are formed on a substrate 1, and a cantilever arm 5 supported by a anchor unit 4 fixed to the substrate is located over the lower electrode 2 and the signal line 3. An upper electrode 6 is formed on the cantilever arm 5, and at the lower of the end portion of the cantilever arm 5, a contact unit 7 is formed for connecting a disconnected portion of the signal line. In the cantilever arm 5 and the upper electrode 6, an intermediate portion is formed narrower than other portions, so that the end portion of the cantilever arm 5 has a constant elasticity.
When a predetermined driving voltage is applied to the upper electrode 6 and the lower electrode 2, the cantilever arm 5 is bended downward due to the electrostatic force generated in the portion of a capacitor structure 8 where the upper electrode 6 and the lower electrode 2 are overlapped with each other, and accordingly, the contact unit 7 connects the disconnected portion of the signal line 3 to perform a switching operation.
FIGS. 2A and 2B are cross-sectional views illustrating an operational state of a cantilever type microelectromechanical switch of the prior art.
The microelectromechanical switch shown in FIG. 1A operates in a single pole double throw (SPDT) scheme. In this microelectromechanical switch, since the signal line 3 and the contact unit 7, respectively connected to an input portion and an output portion, are placed perpendicular to each other and the cantilever arm 5 is supported at one side portion only, when the cantilever arm 5 or the upper electrode 6 is deformed due to thermal expansion during the manufacturing or operation process, the contact between the signal line 3 and the contact unit 7 becomes unstable since the switch cannot move in a vertical direction as shown in FIG. 2A, instead it moves in a bended state as shown in FIG. 2B. This contact degradation increases a contact resistance of the signal line 3 and causes a signal delivery to be unstable, thus reducing the reliability.
FIG. 3 is a perspective view for illustrating an example of a membrane type microelectromechanical switch of the prior art. This membrane type microelectromechanical switch according to the prior art is disclosed in Korean Patent Publication No. 10-0339394.
Two ground planes 41 are formed on a substrate 40 with a predetermined distance apart from each other, two lower electrodes 42 used for a signal line are formed between the ground planes 41. Hinges 44, 45 supported to have a constant elasticity by an anchor 43 are connected to each ground plane 41, and over the lower electrode 42, an upper electrode 46 is located. The upper electrode 46 is connected to be movable upward and downward by the hinge 44 and 45.
When driving voltages are applied to the lower electrode 42 and the ground plane 41, respectively, the upper electrode 46 moves downward by the electrostatic force generated between the lower electrode 42 and the upper electrode 46, and accordingly, lower electrode 42 is connected with each other through the upper electrode 46, to perform a switching operation.
In the membrane type microelectromechanical switch of FIG. 3, the upper electrode 46 serving as a movement plane moves downward by the electrostatic force with the ground plane 41 to connect the lower electrode 42 used for the signal line. Therefore, when the surface of the upper electrode 46 made of a metal during manufacturing process or operation process is deformed by thermal expansion, a problem occurs that the movement plane does not completely contact with the signal line so as to permanently remain open, and stiction occurs between the upper electrode 46 and the lower electrode 42 sustained in a narrow gap, thus reducing the stability and reliability of the switch.
A drawback of this membrane type microelectromechanical switch is the deformation of a membrane and the stiction problem. If the movement plane and the hinges are deformed by the thermal expansion, they cannot move in parallel with the substrate when the movement plane moves by the electrostatic force. This is caused by the fact that since the anchor is fixed to the substrate whose thermal expansion ratio is extremely smaller than the movement plane and the hinge, the movement plane and the hinge are greatly thermal-expanded, while the distance between anchors is not changed. Stress is generated by the thermal expansion in a connection portion between the movement plane and the hinge, in which a permanent deformation is taken place. Consequently, owing to the deformation of the movement plate, problems occur that a normal switching operation cannot be performed when the movement plane becomes abnormally apart from the substrate or is tilted toward one side, and when the movement plane is collapsed near the substrate, that the contact portion of the movement plane permanently contacts with the signal line.
Further, the gap between both electrodes for generating the electrostatic force maintains as close as several micrometers, so that the stiction problem that the driving element adheres to other fixing elements is easy to generate, which acts as significant drawbacks in the operation and reliability of the switch.
As illustrated above, since the conventional microelectromechanical switch is configured in the cantilever type or the membrane type, it has structural problems, such as the thermal deformation and stiction. Such problems have a significant influence on the reliability and the signal isolation feature of the microelectromechanical switch used for improving high insertion loss, low signal isolation, signal distortion, etc.